Use a save editor (rare for DS Zoids), or grind naturally – the game’s difficulty is low-moderate except for post-game arena.
This is a detailed look into (known in Japan as Zoids Saga DS: Legend of Arcadia and sometimes localized as Zoids Legacy DS in fan circles).
L, L, R, R, Up, Down, Up, Down, Left, Left, R, R, Right, Right, Left, Up, START A, A, B, B, L, R, L, R, Up, Up, B, B, Down, Down, Up, L, A